Fitting Dual Pivot internal nutted brake calipers to an early 80's Tange DB steel frame in place of the old single pivot standard nut caliper.

I need to drill out the forward part part of the bridge out to take the internal nut. I don't have a right angle drill attachment, and the ones I have looked at (Bunnings and Supercheap Auto) are a liitle big to get into the space between the seat tube and the seat stays.

Any alternatives or recommendations to enlarge the hole in the bridge, but keeping the original diameter for the caliper bolt on the caliper side of the bridge?

I have no real clues besides an air driven angle headed drill. Sometimes these are quite compact.

Of course you would need a compressor, hose and angle drill and probably shorten the drill bit on a grinder - but who has that equipment and the angle drill would probably be too big to fit in the space anyway.

<<but keeping the original diameter for the caliper bolt on the caliper side of the bridge? >>

Why?
If you just drill a hole the dia of the smallest part of the internal bolt. Then the problem is solved because there is a little increase in dia at the end of the nut. And you have a washer on the other side to hold the caliper steady.
There is no sideways movement possible.
Have done it myself many times.

The bridge on my bike is a plain tube both sides, hence I need one more of those special washers.

Here is the rear of mine

You can see where I need the extra washer

I think your brake blocks are installed upside down

As I understand, the wings on the blocks are guides to help guide the wheel between the blocks when installing the wheel.

UPDATE:

1. Bought some drill bits and a right angle drill adapter

2. The space between the rear bridge and seat tube was the same as that needed by the RA adapter.

3. The RA adapter came with a keyless chuck. I have a 9V Makita Cordless drill wth a smaller length keyed chuck. Swapped chucks - gave me nearly a cm of clearance to be able drill.

4. Selected a 9/32 bit I had a resharpened after breaking the tip off earlier while progressively drilling out the back of the front fork without being held properly in a vice or similar.

5. Took to the selected drill bit and broke it to reduce its length to about 25 mm.

6. Tackled the task to drill the brake bridge out to take the diameter of the recessed nut. Expected that the bridge was not solid but a bridge contained sleeve within it, and was of about 8 mm OD. This was based on some old TANGE frame component sheets of similar date dug up on the web.

7. Drilling all but removed all of the sleeve, but the rearward facing disc on the bridge remained it's original ID.